In 2009, I set up FreeBSD to boot from a ZFS mirrored dataset. This page documents how I did it.


Pentium 4 (i386) system with three 80-GB hard disks: ad2, ad4, and ad6. 3 GB RAM.


  • Have only ZFS.
  • Have a mirror setup in the zpool.
  • Have the system bootable from any of the disks in the mirror.


First I did a standard UFS2 minimal install of FreeBSD 8.0-BETA2 to ad2, csup to RELENG_8 (also known as 8-STABLE), and rebuilt world and kernel with ZFS loader support. I also built a custom kernel with "options KVA_PAGES=512" (necessary for i386, see the ZFS tuning guide) and a more specific configuration for my hardware.

These settings are on the ad2 system (they also need to be on the resulting ZFS system):

echo 'KERNCONF=KRZFSOHM GENERIC' >> /etc/make.conf
echo 'LOADER_ZFS_SUPPORT=YES' >> /etc/make.conf
echo 'daily_status_zfs_enable="YES"' >> /etc/periodic.conf

These need to be only on the resulting ZFS system, not on the UFS system:

echo 'zfs_load="YES"' >> /ohm/boot/loader.conf
echo 'vfs.root.mountfrom="zfs:ohm"' >> /ohm/boot/loader.conf
echo 'zfs_enable="YES"' >> /ohm/etc/rc.conf

ohm is the name of my zpool. Here we go:

# Force the zpool creation due to the disks being slightly different sizes,
# since they are from different manufacturers. ZFS should make use of the
# smaller disk's size for the mirror (according to the ZFS cheat sheet, see
# the references section).
zpool create -f ohm mirror ad4 ad6
zpool set bootfs=ohm ohm
sh -c 'for fs in tmp usr/home var/log; do zfs create -p ohm/$fs; done'
chmod -v 1777 /ohm/tmp
mkdir -v /ohm/dev
cd /ohm && ln -s -w -v usr/home home
cd /ohm/var && ln -s -w -v ../tmp tmp && cd
zfs create -V 6gb ohm/swap
zfs set org.freebsd:swap=on ohm/swap
zfs set checksum=off ohm/swap

# Go to single user mode for safety and speed.
shutdown now

find -d / -not -path /ohm\* -not -path /etc/fstab -not -path /dev\* -not -path /tmp\* -not -path /var/tmp\* -not -path /usr/obj\* | cpio -pmuvd /ohm

echo 'vfs.root.mountfrom="zfs:ohm"' >> /ohm/boot/loader.conf
echo 'zfs_enable="YES"' >> /ohm/etc/rc.conf
echo -e "#Device\t\tMountpoint\tFStype\tOptions\t\tDump\tPass#" > /ohm/etc/fstab

zpool export ohm && zpool import ohm
cp -vi /boot/zfs/zpool.cache /ohm/boot/zfs
zpool export ohm
dd if=/boot/zfsboot of=/dev/ad4 bs=512 count=1
dd if=/boot/zfsboot of=/dev/ad6 bs=512 count=1
dd if=/boot/zfsboot of=/dev/ad4 bs=512 skip=1 seek=1024
dd if=/boot/zfsboot of=/dev/ad6 bs=512 skip=1 seek=1024

zpool import ohm
zfs set mountpoint=legacy ohm
sh -c 'for fs in tmp usr var; do zfs set mountpoint=/$fs ohm/$fs; done'

# Remember to change the BIOS hard drive boot priority setting.

# Add ad2 to the zpool to make it a 3-way mirror.
gpart delete -i 1 ad2
gpart destroy ad2
dd if=/boot/zfsboot of=/dev/ad2 bs=512 count=1
dd if=/boot/zfsboot of=/dev/ad2 bs=512 skip=1 seek=1024
zpool attach ohm ad4 ad2

zfs internal error: failed to initialize ZFS library

This error happens when first trying to use ZFS commands in the FreeBSD livefs environment.


kldload /dist/boot/kernel/opensolaris.ko
kldload /dist/boot/kernel/zfs.ko



None of these references contained precise instructions on how to do exactly what I wanted to do, but they all helped me figure this out.